High-performance blue OLED using multiresonance thermally activated delayed fluorescence host materials containing silicon atoms.

We report three highly efficient multiresonance thermally activated delayed fluorescence blue-emitter host materials that include 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene (DOBNA) and tetraphenylsilyl groups. The host materials doped with the conventional N7,N7,N13,N13,5,9,11,15-octaphenyl-5,9,11,15-tetrahydro-5,9,11,15-tetraaza-19b,20b-diboradinaphtho[3,2,1-de:1',2',3'-jk]pentacene-7,13-diamine (ν-DABNA) blue emitter exhibit a high photoluminescence quantum yield greater than 0.82, a high horizontal orientation greater than 88%, and a short photoluminescence decay time of 0.96-1.93 µs. Among devices fabricated using six synthesized compounds, the device with (4-(2,12-di-tert-butyl-5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracen-7-yl)phenyl)triphenylsilane (TDBA-Si) shows high external quantum efficiency values of 36.2/35.0/31.3% at maximum luminance/500 cd m-2/1,000 cd m-2. This high performance is attributed to fast energy transfer from the host to the dopant. Other factors possibly contributing to the high performance are a T1 excited-state contribution, inhibition of aggregation by the bulky tetraphenylsilyl groups, high horizontal orientation, and high thermal stability. We achieve a high efficiency greater than 30% and a small roll-off value of 4.9% at 1,000 cd m-2 using the TDBA-Si host material.

Graphene-Based Intrinsically Stretchable 2D-Contact Electrodes for Highly Efficient Organic Light-Emitting Diodes.

Intrinsically stretchable organic light-emitting diodes (ISOLEDs) are becoming essential components of wearable electronics. However, the efficiencies of ISOLEDs have been highly inferior compared with their rigid counterparts, which is due to the lack of ideal stretchable electrode materials that can overcome the poor charge injection at 1D metallic nanowire/organic interfaces. Herein, highly efficient ISOLEDs that use graphene-based 2D-contact stretchable electrodes (TCSEs) that incorporate a graphene layer on top of embedded metallic nanowires are demonstrated. The graphene layer modifies the work function, promotes charge spreading, and impedes inward diffusion of oxygen and moisture. The work function (WF) of 3.57 eV is achieved by forming a strong interfacial dipole after deposition of a newly designed conjugated polyelectrolyte with crown ether and anionic sulfonate groups on TCSE; this is the lowest value ever reported among ISOLEDs, which overcomes the existing problem of very poor electron injection in ISOLEDs. Subsequent pressure-controlled lamination yields a highly efficient fluorescent ISOLED with an unprecedently high current efficiency of 20.3 cd A-1 , which even exceeds that of an otherwise-identical rigid counterpart. Lastly, a 3 inch five-by-five passive matrix ISOLED is demonstrated using convex stretching. This work can provide a rational protocol for designing intrinsically stretchable high-efficiency optoelectronic devices with favorable interfacial electronic structures.

Highly Efficient Solution-Processed Organic Light-Emitting Diodes Containing a New Cross-linkable Hole Transport Material Blended with Commercial Hole Transport Materials.

A new small-molecular thermally cross-linkable material {[4-(9-phenyl-9H-carbazol-4-yl)phenyl]-bis-(4'-vinylbiphenyl-4-yl)-amine} (PCP-bis-VBPA, PbV) containing the styrene moiety was synthesized for hole transport layers in wet processed organic light-emitting diodes (OLEDs). It was found that PbV exhibited relatively high glass temperatures above 154 °C and a triplet energy (T1) greater than 2.81 eV. This new synthetic hole transport material (HTM) forms very uniform films after cross-linking reaction with little pin-holes, although it was small-molecule-based cross-linkable HTM. However, to solve the certain minor non-uniformity caused by pinholes with various sizes, a semi-interpenetrating network was formed with well-known polymeric HTM with high mobility [e.g., poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenyl amine), TFB, or poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine, poly-TPD]. As a result, we successfully fabricated red phosphorescent OLED showing an efficiency of about 16.7 cd/A and 12.4% (external quantum efficiency) if we applied PbV blended with 20% of TFB or poly-TPD. In particular, the efficiency and lifetime are significantly improved by 1.5 and 4.5 times, respectively, compared to those of the control device without using blended HTM.

Correction: An indenocarbazole-based host material for solution processable green phosphorescent organic light emitting diodes.

[This corrects the article DOI: 10.1039/D1RA04855D.].

An indenocarbazole-based host material for solution processable green phosphorescent organic light emitting diodes.

We designed and synthesized a new host material with a highly soluble and thermally stable indenocarbazole derivative (7,7-dimethyl-5-phenyl-2-(9-phenyl-9H-carbazol-3-yl)-5,7-dihydro-indeno[2,1-b]carbazole) that can make green phosphorescent organic light-emitting diodes (PHOLEDs) in a solution process. In particular, these are used in a blue common layer structure in which green and red-emitting layers are formed by a solution process and blue common layers are thermally evaporated. The new host material possesses excellent hole transport capability and high triplet energy (T1). Mainly we designed the hole dominant material to keep the exciton forming area away from the hole transport layer (HTL) and emitting layer (EML) interface, an interfacial mixing area to improve device performance. As a result, the greatest lifetime of 1300 hours was achieved and a high current efficiency of up to 66.3 cd A-1 was recorded when we used the optimized device structure of a 5 nm thick bipolar exciton blocking layer (B-EBL). It may be a good agreement of exciton confinement and reduced electron accumulation at the HTL and EML interface.

Improvement of viewing angle dependence of bottom-emitting green organic light-emitting diodes with a strong microcavity effect.

The current efficiency and color purity of organic light-emitting diodes (OLEDs) can be easily improved by means of a microcavity structure, but this improvement is typically accompanied by a deterioration in the characteristics of viewing angle. To minimize the angular dependence of the color characteristics exhibited by these strong microcavity devices, we investigated the changes in the optical properties of the green OLED with a bottom resonant structure. This investigation was based on varying the hole transport layer and semitransparent anode thicknesses. The results of optical simulations revealed that the current efficiency and viewing angle characteristics can be simultaneously improved by adjusting the thickness of the two layers. Furthermore, optical simulations predicted that the angular color dependence could be limited to 0.019 in the International Commission on Illumination (CIE) 1976 coordinate system. This optimum condition yielded a current efficiency of ∼134 cd/A. To further reduce this color shift, a nanosized island array (NIA) was introduced through the dewetting process of cesium chloride. By employing NIAs, the color coordinate shift value was reduced to 0.016 in the CIE 1976 coordinate system, and a current efficiency of 130.7 cd/A was achieved.

Molecular Stacking Effect on Small-Molecular Organic Light-Emitting Diodes Prepared with Solution Process.

The light-emitting layer (EML) is generally prepared by mixing the host and dopant to realize an organic light-emitting diode (OLED). However, phase separation is often observed during the fabrication process to prepare OLEDs, depending on the structure of the host materials. In particular, phase separation because of π-π stacking is frequently observed during thermal annealing for the solution process. The annealing process is required for solvent removal and complete relaxation of the molecule. Hence, the materials with a high glass transition temperature (Tg) are ideal because phase separation occurs because of π-π stacking during the annealing process, if Tg is too low. To understand this phenomenon, we compared two host materials with similar molecular weights but different three-dimensional connectivity, which causes different rotational freedom. Then, we investigated the effect on the device properties, depending on the annealing conditions. In both materials, when the annealing temperature rises above 120 °C, the dopant completely escaped from the EML. However, the material that does not disturb the molecular stacking order by annealing because of its limited free rotation through the internal bond shows much better device characteristics even after annealing at a higher temperature than Tg. The results show that interdiffusion at the interface and unstable internal density distribution with annealing temperature are responsible for the device degradation behavior.

Thermally Cross-Linkable Styrene-Based Host Materials for Solution-Processed Organic Light-Emitting Diodes.

Thermally cross-linkable host materials, DV-TPACZ, DV-TPADBCZ, and TV-TPBI, were designed and synthesized for solution-processed organic light-emitting diodes (OLEDs). The synthesized styrene-functionalized host materials were thermally cross-linked by curing at 150-200 °C without using a polymerization initiator. Excellent solvent resistance was observed for all cured host films. They exhibited low highest occupied molecular orbital energy levels of 5.4-5.7 eV, which indicated a low hole injection barrier from the hole transport layer to the emissive layer. A solution-processed red phosphorescent OLED with 5 wt% (MPHMQ)2Ir (tmd) dopant in the thermally cross-linkable DV-TPACZ host exhibited a current efficiency of 5.3 cd/A, power efficiency of 3.2 lm/W, and external quantum efficiency of 3.6%.

Enhanced device performances of a new inverted top-emitting OLEDs with relatively thick Ag electrode.

To improve the device performances of top-emitting organic light emitting diodes (TEOLEDs), we developed a new inverted TEOLEDs structure with silver (Ag) metal as a semi-transparent top electrode. Especially, we found that the use of relatively thick Ag electrode without using any carrier injection layer is beneficial to realize highly efficient device performances. Also, we could insert very thick overlying hole transport layer (HTL) on the emitting layer (EML) which could be very helpful to suppress the surface plasmon polariton (SPP) coupling if it is applied to the common bottom-emission OLEDs (BEOLEDs). As a result, we could realize noteworthy high current efficiency of approximately ~188.1 cd/A in our new inverted TEOLEDs with 25 nm thick Ag electrode.

Light modulation of top emission organic light emitting diodes showing strong microcavity effect by applying multilayered scattering film.

In order to suppress the viewing angle dependence of top emission organic light emitting diodes (TEOEDs) on a strong microcavity structure, we prepared multi-layered nano scattering film which was consisted with transparent planarizing layer and hazy crosslinked scattering layer. Through such an approach, we could obtain not only a stable color shift and luminance distribution with viewing angle but also a negligible pixel blur level. Meanwhile, we investigated a black tint level of TEOLEDs after attachment of circular polarizer (CP) on various nano scattering films because nano scattering film deteriorates a black level. We found that the black level could be improved from the black tint by reducing the refractive index difference between planarizing layer and scattering layer.

Impact of Interface Mixing on the Performance of Solution Processed Organic Light Emitting Diodes-Impedance and Ultraviolet Photoelectron Spectroscopy Study.

We investigated interfacial mixing of solution-processed organic light-emitting devices (OLEDs) using impedance spectroscopy (IS) and ultraviolet photoelectron spectroscopy (UPS) and its impact on device performance. We focused on interfacial mixing between a solution-processed cross-linkable hole transport layer (XM) and an emitting layer (EML), formed either by solution processing or vacuum evaporation. The results of IS and UPS clearly indicated that extensive interfacial mixing was unavoidable, even after the XM was cross-linked to make it insoluble and rinsed to remove residual soluble species, if the subsequent EML was solution processed. In addition, we also demonstrated that interfacial mixing indeed increased hole current density in corresponding hole only device (HOD). In fact, the hole injection efficiency could be an order of magnitude better when the EML was solution processed rather than vacuum evaporated. We investigated such behavior to find the desirable process condition of solution-processed OLEDs.

Synthesis of Soluble Host Materials for Highly Efficient Red Phosphorescent Organic Light-Emitting Diodes.

New soluble host materials with benzocarbazole and triphenyltriazine moieties, 11-[3-(4,6-diphenyl-[1,3,5]triazin-2-yl)-phenyl]-11H-benzo[a]carbazole and 11-[3'-(4,6-diphenyl-[1,3,5]triazin-2-yl)-biphenyl-4-yl]-11H-benzo[a]carbazole, were synthesized for highly efficient red phosphorescent organic light-emitting diodes (PHOLED). Hole-transporting benzocarbazole moiety and electron transporting triphenyltriazine moiety, which are severely twisted each other enhance the solubility of those materials in common organic solvent. The improved solubility from this molecular design could be due to a reduced π-π stacking interaction, which gives a very uniform film morphology after spin coating of those materials. As a result, we obtained highly efficient soluble PHOLEDs combined with an evaporated blue common layer structure. The resultant red PHOLED exhibited the maximum current efficiency as well as external quantum efficiency values up to 23.7 cd/A and 19.0%.

A nanoporous polymer film as a diffuser as well as a light extraction component for top emitting organic light emitting diodes with a strong microcavity structure.

To improve the viewing angle characteristic as well as the light extraction effect of strong microcavity devices, we fabricated a nanoporous polymer film (NPF) as a scattering medium as well as a light extraction component. We designed two types of organic light emitting diodes (OLEDs) with a strong microcavity effect by changing the thickness of the hole transport layer (HTL; e.g., 30 nm and 60 nm) to investigate two different scattering effects of the NPF. Very interestingly, we could observe a significant enhancement of the external quantum efficiency (EQE) for each device (30 nm thick HTL: 18.0%, 60 nm thick HTL: 31.6%) when we attached a NPF formed on a 125 µm thick PET film coated with the NPF. Furthermore, the NPF successfully suppressed the viewing angle dependence to realize ideal angular emission even in the two extreme microcavity conditions although they are still different from that of a Lambertian distribution.

Highly efficient organic light emitting diodes formed by solution processed red emitters with evaporated blue common layer structure.

We prepared highly-efficient solution-processed red phosphorescent organic light emitting diodes (PHOLEDs) with a blue common layer structure that can reasonably confine the triplet excitons inside of the red emission layer (EML) with the assistance of a bipolar exciton blocking layer. The red PHOLEDs containing EML with a 7 : 3 ratio of 11-(4,6-diphenyl-[1,3,5]triazin-2-yl)-12-phenyl-11,12-dihydro-11,12-diaza-indeno[2,1-a]fluorene (n-type host, NH) : 4-(3-(triphenylen-2-yl)phenyl)dibenzo[b,d]thiophene (p-type host, PH) doped with 5% Iridium(III) bis(2-(3,5-dimethylphenyl)quinolinato-N,C2')tetramethylheptadionate (Red Dopant, RD) produced the highest current and power efficiencies at 23.4 cd/A and 13.6 lm/W, with a 19% external quantum efficiency at 1000 cd/m(2). To the best of our knowledge, such efficiency was the best among those that have been obtained from solution-processed small molecular red PHOLEDs. In addition, the host molecules utilized in this study have no flexible spacers, such as an alkyl chain, which normally deteriorate the stability of the device.

Simple fabrication of a three-dimensional porous polymer film as a diffuser for organic light emitting diodes.

We have investigated a simple and cost-effective fabrication method for a porous polymer film employing the spin-coating process during continuous supply of water droplets by an ultrasonic humidifier. The resulting porous polymer film showed ∼40% optical haze, and this film could be used as a diffuser film for strong microcavity OLEDs. Specifically, we focused on controlling the surface morphology to give a three-dimensional (3D) multi-stacked nanocave structure because we had already learnt that two-dimensional nanoporous structures showed serious loss of luminance in the forward direction. As a result, we found that a 3D-ordered multi-stacked nanocave structure with a relatively small diameter and a distribution range of 300-500 nm can be obtained by precise control of the elastic bouncing behaviour of the supplied water droplets. Using this approach, we found that the 3D nanoporous polymer film can effectively reduce the viewing angle dependency of strong microcavity OLEDs without any considerable decrease in the total intensity of the out-coupled light.

Thermally cross-linkable hole transport polymers for solution-based organic light-emitting diodes.

Two thermally cross-linkable hole transport polymers that contain phenoxazine and triphenylamine moieties, X-P1 and X-P2, are developed for use in solution-processed multi-stack organic light-emitting diodes (OLEDs). Both X-P1 and X-P2 exhibit satisfactory cross-linking and optoelectronic properties. The highest occupied molecular orbital (HOMO) levels of X-P1 and X-P2 are -5.24 and -5.16 eV, respectively. Solution-processed super yellow polymer devices (ITO/X-P1 or X-P2/PDY-132/LiF/Al) with X-P1 or X-P2 hole transport layers of various thicknesses are fabricated with the aim of optimizing the device characteristics. The fabricated multi-stack yellow devices containing the newly synthesized hole transport polymers exhibit satisfactory currents and power efficiencies. The optimized X-P2 device exhibits a device efficiency that is dramatically improved by more than 66% over that of a reference device without an HTL.

New polymeric buffer materials with low driving voltage.

We present excellent polymeric buffer materials based on the poly(9,9-dioctylfluorene-co-N, N-di(phenyl)-N,N-di(3-carboethoxyphenyl)benzidine) (BFE) for highly efficient solution processed organic light emitting diodes (OLEDs). Doped BFE with 3,5-dinitrobenzonitrile (35DNBN), a strong electron acceptor results in significant improvement of current flow and driving voltage. Maximum current- and power-efficiency value of 7.2 cd/A and 5.5 lm/W are demonstrated from blue OLEDs with these doped polymeric anode buffer system. The 40 nm thick anode buffer material showed a similar current density-voltage (J-V) behavior to that of PEDOT:PSS based device. Results reveal a practical way to fabricate the highly efficient solution processed devices for low cost production of printing devices for the future.

Synthesis of a new polymeric host material for efficient organic electro-phosphorescent devices.

We have synthesized a new polymeric host material for phosphorescent dyes, which can be used in phosphorescent light-emitting layers. An alternating copolymer, composed of N-alkylcarbazole and tetramethylbenzene units was synthesized through the Suzuki coupling reaction. We fabricated electro-phosphorescent devices using the synthesized polymeric host doped with solution-processible green and red phosphorescent dyes. Light-emitting devices have an ITO/PEDOT/polymer + dopant/Balq3/Alq3/LiF/Al configuration. The device containing one of two studied green dopants (designated as green 1) in the polymeric host showed the best performance, with a maximum luminous efficiency of 29 cd/A. A thin film of this polymeric was successfully patterned by laser-induced thermal imaging (LITI), and an electro-phosphorescent device was fabricated using the patterned film. This patterned device showed performance characteristics similar to those of a spin-coated device.

Regioselective coupling of pentafluorophenyl substituted alkynes: mechanistic insight into the zirconocene coupling of alkynes and a facile route to conjugated polymers bearing electron-withdrawing pentafluorophenyl substituents.

The reaction of Cp(2)ZrCl(2) with 2 equiv of BuLi at -78 degrees C, followed by the addition of an unsymmetrical tetra- or pentafluorophenyl substituted alkyne R(1)C[triple bond]CAr(f) (R(1), Ar(f) = (CH(2))(4)Me, p-C(6)F(4)H; Me, p-C(6)F(4)H; Ph, C(6)F(5)), resulted in regioselective couplings of these alkynes to zirconacyclopentadienes in which the Ar(f) substituents preferentially adopt the 3,4-positions (beta beta) of the zirconacyclopentadiene ring. With Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) as the zirconocene reagent, the couplings could be carried out at room temperature; however, at higher temperatures significant quantities of the 2,4-fluoroaryl substituted (alpha beta) isomers were also formed. None of the conditions employed produced the 2,5-fluoroaryl substituted (alpha alpha) isomers. These fluoroaryl-substituted zirconacyclopentadienes were readily converted to butadienes via reactions with acids. The zirconacyclopentadiene Cp(2)ZrC(4)-2,5-Ph(2)-3,4-(C(6)F(5))(2), which resulted from the coupling of PhC[triple bond]C(C(6)F(5)), was converted to the corresponding thiophene by reaction with S(2)Cl(2), and to an arene by reaction with MeO(2)CC[triple bond]CCO(2)Me/CuCl. Mechanistic studies on zirconocene couplings of (p-CF(3)C(6)H(4))C[triple bond]C(p-MeC(6)H(4)) indicate that the observed regioselectivities are determined by an electronic factor that controls the orientation of at least one of the two alkynes as they are coupled. Additionally, these studies suggest an unsymmetrical transition state for the zirconocene coupling of alkynes, and this is supported by DFT calculations. The reaction of [(C(6)F(5))C[triple bond]CCH(2)](2)CH(2) with Cp(2)Zr(py)(Me(3)SiC[triple bond]CSiMe(3)) resulted in a zirconacyclopentadiene in which the pentafluorophenyl substituents have been forced into the 2,5-positions (alpha alpha). Zirconocene coupling of the diyne (C(6)F(5))C[triple bond]C-1,4-C(6)H(4)-C[triple bond]C(C(6)F(5)) provided a route to conjugated polymers bearing electron-withdrawing pentafluorophenyl groups.